Cardiac rhythm management devices such as pacemakers, cardioverter/defibrillators, and cardioverter/defibrillators with pacing capability typically include a system for detecting dangerous cardiac arrhythmia conditions in the heart, such as bradycardia, tachycardia, and fibrillation by measuring the time interval between consecutive cardiac depolarizations. Cardiac rhythm management devices receive a sensed cardiac signal comprising electrical activity of the heart and detect cardiac depolarizations in the electrical activity when an amplitude of the electrical activity exceeds a predetermined amplitude level or "sensing threshold." The sensing threshold may be fixed, or may vary over time.
A fixed sensing threshold is not appropriate for detecting certain arrhythmias, such as polymorphic tachycardia and fibrillation, wherein extreme variations occur in the amplitude of the electrical activity during the arrhythmia. The problem of tracking variations in the amplitude of the electrical activity is further complicated when the cardiac rhythm management device delivers pace pulses to the heart, which cause invoked responses which are quite high in amplitude as compared to normal cardiac depolarizations.
One approach to compensate for problems associated with a fixed sensing threshold is to program the sensing threshold at a value determined by the attending physician after careful study of the variety of amplitudes in cardiac signal activity experienced by a patient. In other words, a sensing threshold is programmed into the cardiac rhythm management device, and any cardiac signal amplitude larger than the programmed sensing threshold is considered a cardiac depolarization. If, however, the programmed sensing threshold is set too high and the cardiac signal amplitude decreases significantly, as is often the case in fibrillation, the cardiac rhythm management device may not sense the arrhythmia. If the programmed sensing threshold is set too low, the device may over-sense. For example, a system designed to detect ventricular depolarizations (R-waves) may erroneously detect atrial depolarizations (P-waves) or ventricular recovery (T-waves). Bandpass filtering can be used to partially eliminate erroneous detection of the P-waves and T-waves in a R-wave detection system. If, however, the band of frequencies passed by the bandpass filtering is too narrow, certain fibrillation signals may not be detected.
Another approach to compensate for the above problems is to set the sensing threshold proportional to the amplitude of the sensed cardiac signal each time a cardiac depolarization is sensed. The sensing threshold is then allowed to decrease over time between consecutively sensed cardiac depolarizations so that if the sensed cardiac signal amplitude decreases significantly, the cardiac rhythm management device is still able to detect the lower level amplitude of the cardiac signal. Adjusting the sensing threshold to an appropriate level with this approach becomes difficult if the patient requires pacing due to a bradycardia condition. For example, in a system that senses R-waves according to this approach, the sensing threshold may be adjusted to one-half of the R-wave amplitude when an R-wave is sensed. However, the invoked response due to a first pacing pulse can cause the sensing threshold to be set so high that a second spontaneous R-wave is not sensed. Because the system does not sense the second spontaneous R-wave, a second pacing pulse is delivered to the patient inappropriately.
One solution to the above problem is found in the Kelly et at. U.S. Pat. No. 5,269,300 assigned to Cardiac Pacemakers, Inc., the assignee of the present application. The Kelly et al. patent discloses an implantable cardioverter/defibrillator with pacing capability wherein the sensing threshold is automatically adjusted to a value proportional to the amplitude of the sensed cardiac signal. The sensing threshold continuously decreases between sensed cardiac depolarizations to ensure that a lower level cardiac signal will be detected. However, after a pacing pulse is delivered by the Kelly et al. device, the sensing threshold is set to a fixed value, and held at the fixed value for a predetermined period of time, so that the sensing threshold is not affected by the cardiac response invoked by the pacing pulse. After a predetermined period of time, the sensing threshold is decreased, just as after a spontaneous cardiac depolarization.
In the Keimel et al. U.S. Pat. No. 5,117,824, an R-wave detector automatically adjusts the detecting threshold in response to the R-wave amplitude. The adjustment of the threshold is disabled for a predetermined period following the delivery of each pacing pulse. Thereafter, the sensing threshold is returned to a lower threshold level to allow detection of lower level R-waves indicative of tachyrhythmia conditions.
In the Henry et al. U.S. Pat. No. 5,339,820, a sensitivity control is used for controlling a sensing threshold in a cardiac control device such as a pacemaker, cardioversion and/or cardiac defibrillation device. Initially, a sensing threshold is set to a low value. When the cardiac signal is detected, the amplitude of the R-wave is measured and the sensing threshold is computed as a function of the amplitude of the R-wave. After a refractory period, the sensing threshold is preferably set to 75% of the amplitude of the R-wave. The sensing threshold is then decreased in uniform steps. The uniform steps may be fixed decrements or percentage reductions.
The Gravis et at. U.S. Pat. No. 4,940,054 discloses a cardioversion device having three sensitivities. A first, medium sensitivity is used for the detection of sinus rhythm and ventricular tachycardia. A second, higher sensitivity is designed for differentiating ventricular fibrillation from asystole. A third, lower sensitivity is used to differentiate between R-waves and high amplitude current of injury T-waves which occur after shocking. One of these three sensitivities is selected as a function of the status of the device, such as during a period of suspected tachycardia or a post shock period, and the selected sensitivity must be maintained at least until the next cycle.
The Dissing et at. U.S. Pat. No. 5,370,124 discloses a cardiac rhythm management device having circuitry for automatically adapting the detection sensitivity to the cardiac signal. The detection sensitivity is adjusted by either amplifying the electrical signal supplied to the threshold detector with a variable gain given a permanently prescribed threshold or by varying the threshold itself. In either case, the effective threshold is based on an average value formed over a time interval corresponding to the duration of a few breaths. A switching hysteresis is generated having a lower limit value and an upper limit value, where the threshold is reset only when the average value falls below the lower limit value or exceeds the upper limit value. The limit values of the switching hysteresis are varied with the variation of the threshold, but the relationship of the limit values to the threshold remain unvaried. In one embodiment of the Dissing device, when the threshold is set below a minimum value, a beat-to-beat variance of signal heights of successive input electrical signals are used for forming an average value. The sensing threshold is raised by a predetermined amount if the variance exceeds the predetermined variance value.
The Carroll et al. U.S. Pat. No. 4,972,835 discloses an implantable cardiac defibrillator which includes switched capacitor circuitry for amplifying the cardiac electrical signal with non-binary gain changing steps. Three stages of gain are used to increase the gain approximately 1.5 each increment.
The Baker et al. U.S. Pat. No. 5,103,819 discloses a state machine for automatically controlling gain of the sensing function in an implantable cardiac stimulator. The rate of gain adjustment is dependent on the present sensed conditions and on the prior state of the heart. Different rates of adjustment are selected under varying conditions so that the gain of the sense amplifier is adjusted without significant overshoot. Multiple effective time constants are used for different conditions by basing the rate of adjustment of the sense amplifier gain on the path traversed in the state machine.
Therefore, considerable effort has been expended in providing for automatically adjustable sensing thresholds through adjusting the threshold level itself or with automatic gain circuitry in implantable cardiac rhythm management devices for the purpose of enhancing the capability of the device to sense arrhythmia conditions for which therapy is to be applied.